1. Field of the Invention
The present invention relates to a hydrazone compound, an electrophotographic photoreceptor comprising the hydrazone compound, and an image forming apparatus equipped with the electrophotographic photoreceptor.
2. Description of the Related Art
An electrophotographic image forming apparatus for forming an image through electrophotography (hereinafter referred to as “electrophotographic apparatus”) is much used as duplicators, printers, facsimiles, etc. In the electrophotographic apparatus, an image is formed according to an electrophotographic process as follows:
First, the photosensitive layer of an electrophotographic photoreceptor (this may be hereinafter simply referred to as “photoreceptor”) is uniformly charged to a predetermined potential by charging means such as a charging roller and then exposed to light corresponding to the image information applied thereto by exposure means to thereby form an electrostatic latent image. A developer is applied to the thus-formed electrostatic latent image whereby the toner, a component of the developer is adhered to the surface of the photoreceptor to thereby develop the electrostatic latent image and visualize it as a toner image. Thus formed, the toner image is transferred onto a transfer material such recording paper from the surface of the photoreceptor by transfer means, and then fixed by fixing means. According to the process, an image is formed on a transfer material. On the other hand, the photoreceptor from which the toner image has been transferred is cleaned with cleaning means equipped with a cleaning blade or the like, whereby the toner still remaining on the surface of the photoreceptor not transferred to the transfer material during the transfer operation is removed. Next, the surface of the photosensitive layer is discharged by a discharger, and the electrostatic latent image thereon disappears.
The basic characteristics necessary for the photoreceptor for use in the electrophotographic process are that it has good electric properties, for example, it has good charge-retentive capability and hardly discharges in the dark while it has good photosensitivity and readily discharges through exposure to light. The others also necessary for the photoreceptor are that its electric properties as above are stable even in repeated service so that it can form homogeneous images for a long period of time, and it has good electric characteristic stability (this may be hereinafter simply referred to as “characteristic stability”).
Recently, electrophotography is utilized not only in the field of duplicators but also in other fields of printing plate materials, slide films, microfilms and others for which silver salt photography has heretofore been used, and in addition, it is further applied to high-speed printers using a light source of laser, light-emitting diode (LED), cathode ray tube (CRT) or the like. With the expansion of the application field of electrophotography, requirements for electrophotographic photoreceptors are being high-leveled and broadened.
An electrophotographic photoreceptor is so designed that a photoconductive material-containing photosensitive layer is laminated on a photoconductive substrate. As the electrophotographic photoreceptor, conventionally broadly used is an inorganic photoreceptor equipped with a photosensitive layer comprising, as the main ingredient thereof, an inorganic photoconductive material such as selenium, zinc oxide or cadmium sulfide. The inorganic photoreceptor has basic properties as a photoreceptor in some degree, but has some drawbacks in that the film formation for the photosensitive layer is difficult, the plasticity is not good and the production costs are high. In addition, in general, inorganic photoconductive materials are highly toxic, and are therefore greatly limited in point of their production and use.
As described above, since the inorganic photoconductive material and the inorganic photoreceptor using the same involve many drawbacks, research and development have been progressed for organic photoconductive materials. Further, the organic photoconductive material has been studied and developed generally in recent years and it has been utilized not only for electrostatic recording devices such as the electrophotographic photoreceptor but also has been applied, for example, to sensor devices or organic Electro Luminescent (abbreviated as EL) devices.
The organic using the organic photoconductive material has advantages such that the film formation property for the photosensitive layer is favorable and the flexibility is excellent, as well as it is light in the weight, excellent in the transparency, and a photoreceptor showing good sensitivity to a wavelength region over a wide range can be designed easily by an appropriate sensitizing method. Thus, the organic photoreceptor has been under development as a predominant candidate for the electrophotographic photoreceptor.
In the early days, organic photoreceptors had some drawbacks in point of the sensitivity and the durability thereof, but these drawbacks have been significantly improved by development of a function-separated electrophotographic photoreceptor of which the charge generating function and the charge transporting function are separately attained by different substances. The function-separated photoreceptor of the type has, in addition to the above-mentioned advantages of organic photoreceptors, other advantages in that it has broad latitude in selecting the materials for the photosensitive layer and those having any desired characteristics are relatively readily produced.
The function-separated photoreceptor is grouped into a laminate-structured photoreceptor and a single-layered photoreceptor. In the function-separated photoreceptor of the single-layered function-separated photoreceptor, provided is a single-layered photosensitive layer where a charge generating substance having a charge generating function and a charge transporting substance having a charge transporting function are co-dispersed in a resin called binder resin having a binding function. In the laminate-structured function-separated photoreceptor, provided is a laminate-structured photosensitive layer that comprises a laminate structure of a charge generating layer with a charge generating substance dispersed in a binder resin and a charge transporting layer with a charge transporting substance dispersed in a binder resin.
Various substances have been investigated for the charge generating substance for use in the function-separated photoreceptor, and as those having good light resistance and good charge generating capability, proposed are various materials such as phthalocyanine pigments, squarylium dyes, azo pigments, perylene pigments, polycyclic quinone pigments, cyanine dyes, squaric acid dyes, pyrylium salt dyes.
Further, various compounds have been proposed as the charge transporting material, for example, pyrazoline compounds (for example, refer to Japanese Examined Patent publication JP-B2 52-4188 (1977)), hydrazone compounds (for example, refer to Japanese Unexamined Patent Publication JP-A 54-150128 (1979), Japanese Examined Patent Publication JP-B2 55-42380 (1980), Japanese Unexamined Patent Publication JP-A 55-52063 (1980)), triphenylamine compounds (for example, refer to Japanese Examined Patent Publication JP-B2 58-32372 (1983), and Japanese Unexamined Patent Publications JP-A 2-190862 (1990) and stilbene compounds (for example, Japanese Unexamined Patent Publications JP-A 54-151955 (1979) and JP-A 58-198043 (1983)).
The charge transporting substances must satisfy the following requirements:    (1) they are stable to light and heat,    (2) they are stable to active substances such as ozone, nitrogen oxide (chemical formula: NOx) and nitric acid generated by corona discharging in charging the photoreceptor,    (3) they have high charge transporting ability,    (4) they have high compatibility with an organic solvent and a binder resin, and    (5) they can be manufactured easily and inexpensively.However, while the charge transporting substances disclosed in, for example, the above-stated JP-B252-4188, JP-A54-150128, JP-B2 55-42380, JP-A 55-52063, JP-B2 58-32372, JP-A 2-190862, and JP-A 54-151955, JP-A 58-198043, can satisfy a portion of the demands but have not yet satisfy all of the demands at high level.
Further, in recent years, of the above-stated demands, particularly high charge transporting ability has been demanded for the charge transporting substance. For example, higher sensitivity has been demanded as the photoreceptor characteristics corresponding to the requirement of reduction in the size and high speed operation to electrophotographic apparatuses such as copying machines and printers, and the charge transporting ability of the charge transporting substance has been demanded to improve as means for attaining higher sensitivity of the photoreceptor.
Further, in the high speed electrophotographic process, since the time from exposure to the development is short, a photoreceptor of excellent light responsiveness has been required. In a case where the light responsiveness of the photoreceptor is poor, that is, the decaying speed of the surface potential of the photosensitive layer by exposure is slow, the residual potential rises and is used repetitively in a state where the surface potential is not decayed sufficiently. Therefore, the surface charges at a potion to be erased are not sufficiently erased by exposure to cause deterioration of the picture quality such as lowering of the image density in an early stage. In the function separated type photoreceptor, the charges generated from the charge generating substance upon light absorption are transported by the charge transporting substance to the surface of the photosensitive layer so that the surface charges of the photosensitive layer at a portion irradiated with a light are eliminated. Therefore, the light responsiveness depends on the charge transporting ability of the charge transporting substance. Accordingly, high charge transporting ability is required for the charge transporting substance also with a view point of attaining a photoreceptor having high light responsiveness and capable of forming high quality images also in a high speed electrophotographic process.
Further, high durability of the electrophotographic apparatus is also required. In order to attain the high durability, it is necessary that the electrophotographic photoreceptor has excellent durability to electric and mechanical external force and can operate stably for a long period of time. For example, as to the mechanical durability, durability of the surface layer of the photoreceptor is important. In a case where a photoreceptor is used being mounted on an electrophotographic apparatus, the surface layer of the photoreceptor is inevitably scraped at a portion thereof by a contact member such as a cleaning blade or a charge roller. In a case where the amount of film reduction on the surface layer of the photoreceptor is large, since the charge retainability of the photoreceptor is lowered failing to provide high quality images. Accordingly, in order to attain higher durability of the electrophotographic apparatus, it is demanded for a photoreceptor having a surface layer of high mechanical durability resistant to the contact member, that is, having a surface layer of high printing resistance with less amount of film reduction.
As the charge transporting substances satisfying such requirements, proposed are compounds having both a hydrazone structure and a styryl structure (see Japanese Unexamined Patent Publication JP-A 5-66587 (1993)). However, with the recent increase in the requirements for small-sized, high-speed and durable electrophotographic apparatus, desired is development of charge transporting substances having further better charge transporting capability, and there is room for improvement in the charge transporting substances disclosed in JP-A 5-66587.
Electrophotographic apparatus are desired to be able to provide homogeneous images irrespective of the service environment. Accordingly, photoreceptors are required to have good environment stability in that their properties change a little depending on the fluctuation of the ambient environment such as temperature and humidity. For example, photoreceptors are desired not to cause sensitivity reduction even when used in low-temperature environments. To realize such photoreceptors, charge transporting substances are required to have good charge transporting capability.